1. Field
Exemplary embodiments relate to a lens system which is compact and thin.
2. Description of the Related Art
Digital still cameras (DSCs) including a solid state imaging device such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS), or digital video cameras (DVCs) are widely used. In particular, demands for camera modules having mega-pixels resolution have increased, and cameras having picture resolution higher than 5,000,000 pixels are frequently employed. Imaging optical devices such as DSCs using a CCD or a CMOS, or mobile phone cameras are required to have small size, light weight, and low manufacturing costs. Furthermore, when video communications are performed using mobile phones cameras, the cameras must be small.
As solid state imaging devices have been developed, an exit angle of main light (hereinafter, referred to as an exit angle) is 20° to 25°. Also, a plurality of thin and small-sized lenses have been developed.
In addition, a pitch of a plurality of pixels of a solid state imaging device needs to be less than 1.75 μm, and resolution of a lens system of the solid state imaging device needs to be improved. Furthermore, demands for making the solid state imaging device thin have also increased. However, with respect to the pitch of a plurality of pixels that is less than 1.75 μm, color aberration is not sufficiently corrected, and thus, a desired resolution of the lens system of the solid state imaging device is not easily obtained.
Resolution may be increased even with respect to the pitch of a plurality of pixels that is less than 1.75 μm by correcting color aberration. However, in this case, the overall length of the lens system is increased as compared to a diagonal length of the conventional solid state imaging device, and thus, the demand for miniaturization is not satisfied.
For miniaturization, a back focal length may be reduced. However, when the back focal length is greatly decreased, a space in which glass material may be inserted in a rear portion of an infrared ray (IR) cutoff filter may not be obtained, or an assembling margin may be reduced.
When an exit angle is increased, the solid state imaging device may be easily manufactured with thin dimensions. In this case, a shading characteristic of the lens system may be lowered.
Thus, it is difficult to satisfy both the demand for improved resolution of the lens system and the demand for making the lens system thin.